Insert molding is a widely used method to incorporate functional components such as metal pins, screws, electrical contacts, etc. into a plastic injection molding. These functional components are typically inserted at predetermined locations in the open molding tool which comprises a core side and a cavity side which when brought together or closed define the desired mold shape. After the functional components have been inserted, the tool is closed, and the molten plastic is injection into the cavity. The molten plastic flows around these inserts and after appropriate cooling the part can be removed from the tool with these inserts partly incorporated in the plastic body.
Similar insert molding techniques are used for compression molding of e.g. polyester based Sheet Molding Compound (SMC). Here the functional part is inserted in the core and/or cavity side while the tool, which is heated, is open. Next the SMC is loaded into the tool and, as the tool is closed, the polyester SMC starts to flow under heat and pressure, filling the cavity and flowing around the inserts. After appropriate curing time the part can be removed from the tool with the inserts partly incorporated in the plastic body.
Another technique utilizes the insertion of a preformed metal or plastic skin into the cavity side of the mold and the insertion of devices by means of which the part to be molded is to be fastened to other parts into the core side of the mold. The skin and the fastening devices are then combined with the injection of e.g. structural foam to produce one integral piece such as automotive bumper guards.
Other known techniques involve the molding of thermoplastic material onto or around another component, such as Outsert Molding. This process uses a technique where an object is placed at the parting line between the core and cavity of a tool and actually extends out of the tool. After closing of the tool molten material is injected into the cavity and the portion of the object which is inside the cavity is being covered with plastic.
These known methods of insert molding mentioned above involve the placements of the inserts into the core or cavity side of the mold prior to molding resulting in only a portion of the insert being anchored into the plastic moldings.
One example of actual encapsulation of almost 100% of a core into an injection molding is the so called "Lost Core" molding technique used to produce complicated hollow plastic components such as engine intake manifolds. In this method, a metal alloy is first cast into a shape which resembles the hollow interior of the plastic components to be produced. This metal component or core is then placed into an injection molding tool at a predetermined location and held in place by e.g. pins, moving side cores, or in certain locations by the tool parting line. The gap between the metal components and the cavity wall determines the wall section of the finished plastic part.
Following the securement of the metal component, the tool is then closed and molten plastic is injected between the metal component or core and the walls of the cavity. After appropriate cooling the plastic part with a metal core is removed from the tool and placed into a hot oil bath. The metal core is melted by the hot oil and can escape through the openings formed during the injection molding process where the metal core was held in place in the tool prior to molding. The resulting plastic part is hollow, similar to a blow molding, however more complex and with very smooth inner walls.
A technique for encapsulating prefabricated cores with an injected liquid resin is known under the name of Resin Transfer Molding (RTM). This technique involves the fabrication of e.g. a foam core surrounded by several fiberglass mats. This prefabricated core is placed into a tool. The tool is closed and a liquid resin such as epoxy resin is injected into the space provided between the fibers of the fiberglass mat to totally encapsulate the foam core. Sometimes vacuum is applied to the tool, assisting the flow of the resin and the complete wet out of the fiberglass mat. After curing of the epoxy resin, the part can be removed from the tool. This technique is used, for example to produce lightweight and stiff automotive hoods.
These known techniques directed at encapsulating a core or insert involve a mechanical means to keep the core in a predetermined distance from the cavity walls by means of e.g. pins (Lost Core) or a fiberglass mat (RTM) which enables the core to be almost completely encapsulated. It is the object of the present invention to enable the complete encapsulation of a core or insert without the use of pins or other locating means.
DE-A-430144 discloses a method of encapsulating a core or blank by a first molding operation in which the blank is supported on a first mold part matching its shape and its exposed upper surface covered by a layer of plastic injected into a second mold part having a greater perimeter than the blank to provide a projecting plastic border around the blank perimeter. Then in a subsequent second molding operation using a third mold part having a perimeter greater than the blank, the undersurface of the blank is covered by a layer of plastic which extends beyond the perimeter of the blank to again provide a plastic border to butt against the plastic border produced in the fist molding operation. Such a method requires two molding operations using different tooling to effect the encapsulation and results in a butt joint around the perimeter of the blank.